This invention relates to cathode ray tubes (CRTs) and, in one embodiment, to white phosphors for use in such tubes. The term "white" phosphor as used herein means a material which has a cathodoluminescence spectrum including a plurality of spectral lines (wavelengths) in the visible such that the integrated effect is that of white light. The appearance of a light source is defined by the Standard Illuminants established in 1931 by the Commission Internationale de l'Eclairage (C.I.E.). Quantitatively, the color of a phosphor's emission is plotted as an (x,y) coordinate on a C.I.E. chromaticity diagram.
A CRT phosphor may be either a crystalline powder, an amorphous form, or a single crystal. Powders are commonplace and have formed the basis of the CRT (television, monitors, etc.) industry for decades. Although powder phosphors have good light extraction qualities, they have limited resolution due to a light scattering by the powder particles and limited brightness due to thermal loading effects. In the powder phosphor industry white light emission is produced by physically mixing different phosphors which emit the primary colors. Examples of such mixtures include those designated P4, P18 and P22 by the Electronics Industries Association, TEPAC Publication No. 116 (1980). However, this approach suffers from a lack of uniformity which is particularly deleterious in high resolution applications where small grain size is important. In addition, because each pixel must include three grains, one of each color phosphor, the grain sizes must be smaller for a given resolution. Yet, smaller grains have lower cathodoluminescence efficiency.
Very little work has been reported on amorphous phosphors.
A single crystal phosphor, on the other hand, has the unique ability to meet the requirements of small format, high resolution displays because of the absence of light scattering within the single crystal. By comparison, a powder phosphor target for use in a high resolution CRT display has about half the resolution of its single crystal analog. Notwithstanding these advantages of single crystal targets, the prior art has failed to demonstrate a practical single crystal white phosphor material. For example, J. M. Robertson et al have reported in Philips Journal of Research, Vol. 35, p. 354 (1980) the LPE growth of epitaxial YAG phosphors doped with a single trivalent activator (e.g., Tb, Eu, Ce, Sm, Pr, Dy or Tm) that produces emission at specific visible lines. However, simultaneous emission (i.e., co-emission) from two or more activators present together in YAG is not disclosed. Moreover, such co-emission is not obvious from the demonstration of emission of a single activator in YAG because it is frequently the case that the presence of a second activator quenches emission from the first activator and/or conversely, so that emission from both activators may be inhibited. We have found that this quenching phenomenon occurred, for example, when Cr was added to Ce:Bi-doped YAG in an attempt to create a white phosphor. Even though Cr-doped YAG by itself produces red emission and Ce:Bi-doped YAG produces blue-green emission, the addition of Cr to Ce and Bi in YAG quenched the emission of all three. Thus, the prior art has failed to demonstrate not only co-emission from a YAG phosphor but also white light emission from a YAG phosphor. Yet, many applications such as radiology and topography utilize a black and white format which requires a white phosphor.
Other prior art approaches to achieving white light emission rely on a penetration tube design. In one design, as described by T. E. Clark et al, Journal of the Electrochemical Society, Vol. 129, No. 7, p. 1540 (1982), each phosphor particle is coated with a nonluminescent layer which in turn is coated with a different phosphor layer. The e-beam energy is controlled so that for one energy level both phosphors emit light whereas for a lower energy level only the outer phosphor emits light. The efficiency of this design is low, however, because of the nonluminescent layer which absorbs some of the e-beam energy but does not emit light. In addition, the coated particles are difficult to manufacture. In another design described by T. G. Maple et al, Journal of Vacuum Science and Technology, Vol. 10, No. 5, p. 616 (1973), the target includes two or more sputtered phosphor layers which are stacked on top of one another. The layers are simultaneously excited by the electron beam so that a white color appears to be emitted by the CRT. This multiple layer target design has several serious manufacturing problems, however. First, a minimum of two layers must be formed which can seriously reduce the yield, as compared to targets which have only a single phosphor layer. Second, the deflection of the electron beam across the target yields a length, L, over which the energy is dissipated given by L=d/cos .theta., where d is the vertical depth into the phosphor and .theta. is the angle of incidence measured from the normal to the surface. .theta. can range from about 30 degrees for a 25 mm diameter CRT to 45 degrees for a 75 mm diameter display. The result is that more energy is dissipated in the top layer as the beam sweeps to larger radii, resulting in a shift of the color away from the white at the center of the display.
The preceding prior art work on powder mixtures and penetration tube designs utilized multiple phosphors in their attempts to achieve white light emission. However, examples of single phosphors which produce white emission are the Pr-activated garnet powders described by B. J. Green et al, Extended Abstracts of the Electrochemical Society, Las Vegas Meeting Oct. 14-18, 1985, but the only Pr-activated YAG powder produced a reddish-pink color (not white light) as indicated by the wavelength and intensity information of Table 1 and the C.I.E. coordinates of FIG. 1 of the abstract.